The present invention relates to inkjet printing devices, and particularly although not exclusively to a method for improving image quality on plots.
lnkjet printing mechanisms may be used in a variety of different printing devices, such as plotters, facsimile machines or inkjet printers. Such printing devices print images using a colorant, referred to generally herein as xe2x80x9cink.xe2x80x9d These inkjet printing mechanisms use inkjet cartridges, often called xe2x80x9cpens,xe2x80x9d to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms carry an ink cartridge with an entire supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as xe2x80x9coff-axisxe2x80x9d systems, propel only a small ink supply with the printhead carriage across the printzone, and store the main ink supply in a stationary reservoir, which is located xe2x80x9coff-axisxe2x80x9d from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the printhead cartridge. In multi-color cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color also being referred to herein as a xe2x80x9cpenxe2x80x9d.
Each pen has a printhead that includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporisation chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energised to heat ink within the vaporisation chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energised resistor.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energising the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located side-by-side on the printhead, parallel to one another, and substantially perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The height of this band is known as the xe2x80x9cswath heightxe2x80x9d of the pen, the maximum pattern of ink which can be laid down in a single pass.
The orifice plate of the printhead, tends to pick up contaminants, such as paper dust, and the like, during the printing process. Such contaminants adhere to the orifice plate either because of the presence of ink on the printhead, or because of electrostatic charges. In addition, excess dried ink can accumulate around the printhead. The accumulation of either ink or other contaminants can impair the quality of the output by interfering with the proper application of ink to the printing medium. In addition, if colour pens are used, each printhead may have different nozzles which each expel different colours. If ink accumulates on the orifice plate, mixing of different coloured inks (cross-contamination) can result during use. If colours are mixed on the orifice plate, the quality of the resulting printed product can be affected. For these reasons, it is desirable to clear the printhead orifice plate of such contaminants and ink on a routine basis to prevent the build up thereof. Furthermore, the nozzles of an ink-jet printer can clog, particularly if the pens are left uncapped in an office environment.
In an off-axis pen, life goal is on the order of 40 times greater than a conventional non off-axis system, e.g. the printhead cartridges available in DesignJet(copyright) 750C color printers, produced by Hewlett-Packard Company, of Palo Alto, Calif., the present assignee. Living longer and firing more drops of ink means that there are greater probability that the printer print quality degrade and/or deviate along life. This requires finding better ways to keep functional and stable our printheads during long periods and large volumes of ink fired.
In order to maintain the quality of the printed output of the printer device it is important to improve the certainty that each instruction to the printhead to produce an ink drop from a nozzle of the plurality of nozzles does will produce such an ink drop (i.e. good servicing of the printhead and replacing nozzles out with working nozzles in performing error hiding).
In the present application, the term plot means a printed output of any kind or size produced by a printing device. For instance a plot could be a printed CAD image or a printed graphic image like a photo or a poster or any other kind of printed image reproduction.
In U.S. Pat. No. 5,455,608 it is described how a printer may adjusts servicing of the pen based on the result of the current drop detection step only. Before starting a plot these printers perform a drop detection on all the pens to detect if there are any non-firing nozzles (xe2x80x9cnozzles outxe2x80x9d). If a single nozzle out is detected in a pen, the printer triggers a so called automatic recovery servicing process for servicing the malfunctioning pen to recover the malfunctioning nozzle(s).
This process includes a sequence of 3 nozzle servicing or clearing procedures of increasing severity which are performed in sequence so long as some of the nozzles of the printhead fail to fire ink drops pursuant to ink firing pulses provided to the printhead or until all of the procedures have been performed.
At the end of each of these procedures a new drop detection is performed on the pen, to verify if the pen is fully recovered. If, according to the current result of the drop detection, it is not, the subsequent servicing procedure is performed. If, at the end of the 3 functions, the pen is still not fully recovered (i.e. at least one nozzles is still out) the user is reported to replace the pen or to disable the nozzle check. One big drawback of this system when implemented, e.g. as in DesignJet(copyright) 750 C printers, is that if the printer is not able to fully recover the failing nozzles or there are some unstable nozzles, the system will remain in this recovery servicing mode until the decease of the printhead, being forced, by the permanent nozzle out, to run this process at the beginning of each plot. This usually leads to either an unacceptable loss of throughput and printer productivity (because the printer stops and waits for an answer, the automatic recovery process is very time consuming, and causes a big waste of ink particularly when running the priming functions) or to excessive printhead replace or continue messages that users disable nozzle check via front panel, causing throughput losses.
European Patent Application no. 99 103283.0 in the name Hewlett-Packard Company (Docket number 60980059) describes a technique for servicing a printhead, by checking the status of the printhead by means of a drop detector sensing ink droplets fired by the nozzles of such a printhead. This technique monitors the more recent status of the nozzles and employs an incremental counter, reporting in a condensed way a number of historical statuses of the nozzles, to decide whether or not executing a recovery service on the printhead. In particular the recovery algorithm comprises 3 different servicing procedures (spitting, wiping, priming) which are applied in sequence, from the softer servicing (spitting) to the stronger one (priming), to the printhead. The decision to pass from one servicing procedure to the next one in the sequence is based on the monitored efficacy of the currently applied servicing procedure, i.e. if a servicing procedure is increasingly recovering nozzles, this is usually repeated; if not, a stronger servicing procedure is started to attempt the recovery of the still malfunctioning nozzles. However, monitoring only the efficacy of a servicing procedure, implies the fact that some non-efficacious procedures (sometime these may affect the lifetime of the printhead itself) are often performed and than abandoned. The performance of useless, or even damaging, servicing procedures is then increasing the length of the entire recovery algorithm. In addition such unneeded recoveries may generate wear in the nozzle plate and in the component of service station and possibly a waste of ink. Finally the execution of wrong servicing may generate additional defects in the printhead.
The specific embodiments and methods according to the present invention aim to improve the efficiency and the efficacy of the recovery process thereby improving printing quality and the functional lifetime of the plurality of nozzles.
According to an aspect of the present invention, there is provided a method of improving image quality on plots produced by a printhead, which has a plurality of nozzles, mounted in an inkjet printing device for printing plots, each nozzle having at least a working status and a failing status and such printing device is capable of performing a variety of functions to improve image quality, said method comprises the steps of: (a) checking the status of one or more nozzles; (b) storing in a memory support the status of a checked nozzle as detected during said checking step; and (c) based on a plurality of said statuses stored over time in said memory support, performing an appropriate function form said variety for improving the image quality.
The fact that data on historical statuses nozzles are stored in a memory support allows to better evaluate what sort of functions can be executed to improve image quality. Generally error hiding techniques base their generation of print masks taking into account the current status of the nozzles only, e.g. when using printed test patterns, either automatically or manually checked. The capability of executing a process(s) or function(s) based on how the status of the nozzle(s) changed over time, gives great flexibility and accuracy in selecting the one which can achieve an higher image quality on plots, fitting with the current health of the printhead.
Preferably, a nozzle in the failing status comprise a malfunctioning nozzle or an aberrant nozzle.
In a preferred embodiment, the variety of functions comprises (i) one or more error hiding function for replacing nozzles in a failing status with nozzles in a working status while printing plots and (ii) one or more servicing functions for recovering a nozzle in failing status back to a working status.
In this way, i.e. applying error hiding or servicing functions, the method is trying to control and improve the failures, affecting the image quality, which are less stable during the life of a printhead.
Typically, one or more servicing functions are applied in sequence if the nozzle is still in a failing status and after applying one or more servicing functions, one or more error hiding functions are also applied to hide a nozzle still in a failing status.
Accordingly, performing different level of functions, depending on the persistency of the failure, allows to reduce the waste of time in the servicing and to control the wear of the nozzle plate by not applying non-required functions.
More preferably, The method further comprises the step of identifying the cause of failure of a nozzle in a failing status, before the step of performing said appropriate function.
The identification of what is causing the failure of the printhead allows to improving the efficiency and efficacy of the recovery process. Firstly, an appropriate recovery can be often identified before executing any additional recovery functions, so speeding up the entire process. Secondarily, by allowing to skip the unnecessary functions and to apply only the ones that are more likely to solve or improve the failure, this can reduce most of the problems generated by the execution these unneeded or wrong functions.
Preferably, the step of identifying comprises the step of observing how the status of the nozzle is changing over time. Advantageously, said step of identifying the cause of failure of a nozzle in a failing status is based on examining said plurality of statuses individually stored over time in said memory support.
Contrary to what suggested in the EP Application no. 99 103283.0 cited above, the collection of data relative to the failures is now stored individually and not incrementally, in order to gives to a pattern recognition algorithm enough details over the previous statuses of the nozzles. This allows to track the evolution of the failure and so an easier identification of the possible causes of the defect(s) of the nozzle(s) or the printhead.
Viewing a second aspect of the present invention, there is also provided a computer program which comprises computer program code means performing the following steps when said program is run on an inkjet printing device comprising a printhead, having a plurality of nozzles, and said printing device being capable of performing a variety of functions for improving image quality: (a) enabling the device to check the status of one or more nozzles; (b) storing in a memory support the status of a checked nozzle as detected during said checking step; and (c) based on a plurality of said statuses stored over time in said memory support, enabling the device to perform an appropriate function for improving the image quality.
Viewing a forth aspect of the present invention, there is also provided an inkjet printing device for printing plots which comprises a printhead, having a plurality of nozzles, a servicing unit capable of applying recovery functions to said plurality of nozzles, detection means for checking if a nozzles is in a working status or in a failing status, memory means and a plurality of functions executable by said device to improve image quality; said memory means, responsive to said detection means, contains data on how the status of a nozzle is varying over time and said device further comprises means to select and execute at least one of said plurality of functions, responsive to the data stored in said memory means.